The invention relates generally to wireless communication and, more particularly, to the control of transmit power in a spread spectrum wireless communication system.
A widely used technique for wireless communication is code division multiple access (CDMA) signal modulation. A CDMA system may be designed to support one or more CDMA standards such as (1) the xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the xe2x80x9cTIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Stationxe2x80x9d (the IS-98 standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the WCDMA standard), (4) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cTR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005xe2x80x94A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 CDMA2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the CDMA2000 standard), and (5) other standards.
In a CDMA system, multiple communications are simultaneously transmitted between base stations and mobile subscriber units over a spread spectrum radio-frequency (RF) signal. In CDMA and other spread spectrum systems, maximizing system capacity and maintaining quality of service are paramount concerns. System capacity in a spread spectrum system can be maximized by carefully controlling the transmit power of each subscriber unit and base station in the system.
If a signal transmitted by a subscriber unit arrives at the base station at a power level that is too low, the bit-error-rate may be too high to permit effective communication with that subscriber unit, undermining quality of service. On the other hand, signals with power levels that are too high can interfere with communication between the base station and other subscriber units in the system, reducing system capacity. For this reason, to maintain system capacity and quality of service, it is desirable to optimize the level of transmit power of signals transmitted by base stations and subscriber units.
The subscriber units and base stations typically communicate with one another to control and optimize the transmit powers of signals sent in the system. For example, to control the level of transmit power in a subscriber unit, the base station estimates the power of a signal received from the subscriber unit and transmits instructions, such as power control bits, to the subscriber unit via a control channel in the downlink. The downlink is sometimes called the xe2x80x9cforward link.xe2x80x9d Similarly, to control the level of transmit power in a base station, a subscriber unit estimates the power of a signal received from the base station and transmits power control bits to the base station via a control channel in the uplink. The uplink is sometimes called the xe2x80x9creverse link.xe2x80x9d In each case, the control channel may coincide with a pilot channel. The base station adjusts transmit power based on the power control bits transmitted from the subscriber unit. Likewise, the subscriber unit adjusts transmit power based on the power control bits transmitted from the base station.
The invention is directed to the control of transmit power in a spread spectrum wireless communication device when all of the demodulation elements, or xe2x80x9cfingers,xe2x80x9d of a rake receiver associated with the device are xe2x80x9cout-of-lock.xe2x80x9d For example, the invention may be used to control the power of a signal transmitted by a base station when all of the fingers assigned to track a particular subscriber unit signal are out-of-lock. Likewise, the invention may be used to control the power of a signal transmitted by a subscriber unit when all of the fingers assigned to track a particular base station signal are out-of-lock. The invention is particularly useful in WCDMA communication systems, but may have wide application to a variety of systems.
The fingers of a rake receiver are used to track multiple received paths of a spread spectrum signal in a multi-path environment. An xe2x80x9cout-of-lockxe2x80x9d condition occurs when the output of a finger in the rake receiver drops below a predetermined signal strength. When all fingers are out-of-lock, the base station or subscriber unit is unable to receive the signal from the other device. Thus, power control techniques can be ineffective because the power control bits are not available for adjustment of transmit power in the receiving device. The invention applies a modified power control routine to the transmitter of a device when all of the fingers assigned to track a particular signal from another device are out-of-lock. In this manner, more effective power control in the systems can be achieved.
The out-of-lock condition may be caused by fast fading or slow fading. Fast fading may result from momentary cancellation of multipath signals, and often affects only the uplink or downlink, but not both. Slow fading may result from a large obstruction or dense array of obstructions that affect both the uplink and downlink. The uplink and downlink typically occupy different frequencies. Consequently, when there is fast fading in the uplink, the downlink is usually operable, permitting transmission of the power control bits to the xe2x80x9cfadingxe2x80x9d device. Lock is often regained as the fading device adjusts its transmit power in response to the power control bits in the downlink. Therefore, in the case of fast fading, it is assumed that the fading device continues to receive the control channel, and is capable of responding to the power control bits to overcome the fading condition.
When the out-of-lock condition is due to slow fading, however, both the uplink and downlink may be inoperable. In other words, the fading device may be incapable of receiving the control channel from the other device, and cannot rely on power control bits for adjustment of transmit power to overcome the fading condition. In that case, if the out-of-lock condition was detected by the base station, it can be advantageous to increase the transmit power of the base station in an effort to transmit the control channel over the downlink to effect power control in the subscriber unit and thereby regain lock. The same may be true when the out-of-lock condition is detected in a subscriber unit.
The invention involves classifying an out-of-lock condition as either a fast fading condition or a slow fading condition. When the out-of-lock condition is classified as a fast fading condition, the invention maintains transmit power in the device that experienced the out-of-lock condition. When the out-of-lock condition is classified as a slow fading condition, however, the invention increases transmit power in the device that experienced the out-of-lock condition.
The invention classifies an out-of-lock condition as either a fast fading condition or a slow fading condition based on the duration of the out-of-lock condition. When all fingers are out-of-lock in the base station, for example, the base station initially assumes a fast fading condition and maintains its transmit power at a substantially constant level. When all of the fingers remain out-of-lock for an extended period of time, however, the base station assumes a slow fading condition and increases its transmit power in an effort to reacquire a lock with the signal transmitted by the subscriber unit. The invention may implement a timer, for example, to facilitate classification of out-of-lock conditions.
As transmit power is increased in the device that detects the out-of-lock condition, there is a better chance that the fading device will receive the power control bits in the control channel and increase its own transmit power. In this manner, the invention can help avoid prolonged out-of-lock conditions, and thereby promote quality of service. Transmit power is only increased when the fingers remain out-of-lock for an extended period of time, however, avoiding undue increases in transmit power that could produce interference among subscriber units.
In one embodiment, the invention provides a method comprising detecting when all demodulation fingers in a wireless rake receiver are out-of-lock, and increasing a level of transmit power of a wireless transmitter when the demodulation fingers remain out-of-lock for more than a predetermined period of time.
In another embodiment, the invention provides an apparatus having a rake receiver with demodulation fingers that receives wireless signals, a transmitter that transmits wireless signals and a detector that detects when all of the demodulation fingers in the rake receiver are out-of-lock. The invention may also include a controller that increases a level of transmit power of the transmitter when the demodulation fingers remain out-of-lock for more than a predetermined period of time.
In an added embodiment, the invention provides a method comprising detecting when all demodulation fingers in a wireless rake receiver are out-of-lock and classifying the out-of-lock detection as due to a fast fading condition or a slow fading condition in the received signal. The method may also include increasing a level of transmit power of a transmitter in the event the out-of-lock detection is classified as a slow fading condition.
In a further embodiment, the invention provides an apparatus comprising a rake receiver with demodulation fingers that receive wireless signals, a transmitter that transmits wireless signals, and a detector that detects when all demodulation fingers in the rake receiver are out-of-lock. A controller classifies the out-of-lock detection as due to a fast fading condition or a slow fading condition in the received signal. In the event the out-of-lock detection is classified as a slow fading condition, the controller increases a level of transmit power of a transmitter.
The invention can provide a number of advantages. For example, the invention can be used to increase transmit power to assist in more quickly reacquiring lock with another communication device in the case of a slow fading condition, thereby promoting quality of service for mobile subscribers. At the same time, the invention can increase transmit power on a selective basis to avoid indiscriminate increases that could create excessive interference and adversely impact system capacity. In particular, by increasing transmit power only when the out-of-lock condition appears to be the result of slow fading, the invention more effectively balances interests in both quality of service and system capacity. Accordingly, with improved management of transmit power, the invention may be effective in reducing the number of dropped users and increasing overall system capacity.